The purpose of epicyclic trains is, inter alia, to permit extremely large reduction ratios to be achieved with few wheels. With steplessly adjustable friction gears, as the name implies, it is possible to select different output speeds at constant input speeds, or vice versa.
German Pat. No. 374,112 describes a steplessly adjustable friction transmission wherein the planet wheels have two crowned running surfaces and are forced against the inside surfaces of two outside sun wheels by centrifugal force only. As a result, extremely high driving speeds are necessary to obtain acceptable output torques. Also, there is no advantageous utilization of the resolution of forces by wedging action. In the various embodiments, the transmission ratio is changed not by shifting the shafts of the planet wheels parallel to the running surfaces, but by tilting these shafts, so the crowned running surfaces are indispensable. This manner of adjustment is sufficient only for contact pressures caused by centrifugal forces; it is not suited for trains with only one planet wheel whose shaft is shifted parallel to the running surface.
German Auslegeschrift (DT-AS) No. 1,231,080 shows such a steplessly adjustable epicyclic friction gear wherein the single planet wheel is radially shiftable and has conical friction surfaces, but there the necessary contact pressure must be exerted also by springs, hydraulic and/or electrical devices, i.e., not simply positively mechanically as a function of the speed of the input shaft.
Mechanical, torque-dependent pressure-exerting devices are known in the art. British Pat. No. 1,283,375, for example, discloses a mechanical pressure-exerting device wherein a cam with curves acting in both directions of rotation is disposed on the drive shaft of the gearing. According to the direction of rotation, one of these cam curves presses on the shaft of an intermediate wheel which rolls around the inside of a planet wheel, the two wheels being maintained in frictional driving contact with each other. The planet wheel, in turn, rotates in a cylindrical outer ring, the planet wheel and the outer ring being in frictional driving contact with each other. Such high-reduction-ratio transmissions are usually driven by high-speed electric motors with correspondingly low torque. The arrangement with an intermediate wheel turns out to be expensive and possibly unreliable because the speed of the intermediate wheel is very high. This may also result in an unduly high noise level.